This revised proposal builds on our exciting preliminary findings demonstrating the power of a mitochondrial targeted antioxidant to protect skeletal muscle from zidovudine (AZT)-induced dysfunction. Due to the success of HIV treatments, which often includes AZT, over 35% of all HIV patients in the US are 50 years and older. New data demonstrates that AZT-induced changes are very different in young and old skeletal muscle. However, the interactions between these life-saving therapies and aging on mitochondria remain poorly understood. AZT and other nucleoside reverse transcriptase inhibitors (NRTl) have significant mitochondrial toxicity that shares many characteristics with aging muscle, including increased mitochondrial mutations, reduced mitochondrial content, impaired energy production, and increased oxidative stress leading to muscle loss and frailty. Despite these similarities, there have been no studies examining the potential synergistic effects of aging and NRTl treatment on mitochondrial function in skeletal muscle. In this proposal, we use a common NRTl combination, AZT/3TC (combivir), to test whether aging exacerbates NRTI-induced mitochondrial toxicity and whether mitochondrial targeted antioxidants can prevent this mitochondrial dysfunction. Aim 1 (Function) uses state of the art spectroscopic and traditional approaches to test whether aging worsens the functional decline in AZT/3TC treated muscle and whether mitochondrial targeted catalase (mCAT) is able to prevent this decline. Aim 2 (Mechanism) elucidates the cellular mechanisms underlying AZT/3TC toxicity and mCAT protection in young and old mouse muscle. We combine methods developed during the last P01 grant cycle for measuring changes in the mitochondrial proteome, damage, and quality control processes. Aim 3 (Translation) tests the potential for translating the protective effects of the transgenic mCAT model to human application. We test whether newly developed mitochondrial targeted antioxidant peptides (SS peptides) can protect against AZT/3TC toxicity in young and old mouse muscles. We combine methods from Aims 1 and 2 to measure in vivo functional outcomes and underlying mechanisms of protection of the SS peptides.